Antireflective films or layers are commonly employed on surfaces of such optical devices as lenses, windows, filters and the like. One area of recent interest, requiring a method of greatly reduced cost and complexity, is the application of antireflection films to glass envelopes for solar energy collectors. The reduction of reflection losses occurring at the collector envelope can substantially increase collector efficiency.
In general, the reflection of light from a surface depends not only on the angle of incidence of the light, but also upon the refractive index of the material forming the surface. For a glass having a refractive index of about 1.5, the reflectance from a single surface is about 4% for light impinging at incident angles of less than about 50.degree. from the normal.
There presently exists a highly developed technology for producing antireflective layers on the surfaces of optical devices. Heretofore, the highest quality coatings have been produced by vacuum deposition techniques. While such techniques are suitable for the batch treatment of small articles such as lenses, they are expensive and not readily adaptable to the continuous production of antireflective layers on large articles. Particular difficulty is encountered in attempting to provide antireflective layers on surfaces of articles of complex shape such as the interior walls of tubular glass envelopes for solar energy collection devices. Single-layer coatings which are simple to apply, are adequate in some applications, but they have some serious limitations. The lowest reflectance that can be attained for crown glass, for example, using practical materials, is 1.33%. Moreover, the low reflectance property is operative over a narrow band of wavelengths and rises rather sharply at wavelengths that are longer and shorter than that of the minimum reflectance. Multiple layer films have usually been employed to provide low reflectance over a broader band of wavelengths. However, the processes by which such multiple layer films are deposited are costly and complex, fifteen or more layers being required to form some commercial broad band antireflective films. Also, the transmittance of the optical device decreases as the number of layers increases.
It has been known that a single layer having a gradient refractive index can provide broad band antireflective properties. Attempts have been made to produce such a surface layer by techniques whereby leachable components are removed from the glass, leaving a skeletonized porous surface having a lower refractive index than the bulk glass. In accordance with such prior art techniques, etching is permitted to proceed to an extent sufficient to provide a skeletonized surface layer of depth approximating an odd multiple of one-fourth the wavelength of the light to be transmitted to reduce reflectance of that light by the treated surface.
Most of the known etching processes involve complex etching solutions and procedures which must be designed specifically for the type of glass composition to be treated. U.S. Pat. No. 2,348,704 to Adams, for example, describes a procedure for treating barium crown glass by removing the alkali, alkaline earth, and other bivalent metal oxides from the glass, and thereafter treating the glass with hydrofluoric acid to enlarge the pore structure of the residual siliceous layer. A single leaching step is taught in U.S. Pat. Nos. 2,486,431 to Nicoll et al. and 2,490,662 to Thomsen which describe methods for treating soda-lime glasses or optical crown glasses with complex, silica-saturated solutions of fluosilicic acid, in order to provide antireflective surface films thereon. The Nicoll et al. and Thomsen etch solutions are complex, and if the solution is not sufficiently saturated it will completely dissolve the glass surface. Moreover, if it is over saturated, it will lay down a coat of SiO.sub.2 on the glass surface. It is stated in the aforementioned U.S. Pat. No. 2,490,662 that the treating solution dissolves out of the surface of the glass substantially all of the metallic oxides and some of the silica thereby producing a zone very shallow in depth consisting of silica molecules. It is further stated that it appears that silica is simultaneously dissolved from the glass surface and redeposited at spaced points. The solutions are subject to rapid chemical change, hence requiring constant careful monitoring. In general, prior art etching processes resulted in surface films which had low abrasion, weather and chemical durabilities. These methods are not effective to produce efficient antireflective surface layers on durable glasses such as borosilicate glasses. As noted by L. Holland in The Properties of Glass Surfaces, Wiley & Sons, New York, (1964) on pages 155 and 165, acid etching processes did not produce antireflective films on the surfaces of chemically-durable borosilicate glasses. Holland points out that the production of antireflective films by chemical etching was deemed of little practical value in view of the weak and optically inefficient nature of the films so produced. A further disadvantage of prior art processes is the inability to control the optical and physical characteristics of the film by controlling such parameters as the shape, size and density of pores in the surface layer.